The present invention relates to the drafting and condensing of a roving of textile fibre before its transformation into twisted yarn. Hereinafter it is described with reference to ring spinning although the invention can be utilised advantageously also in other applications of the textile industry.
Ring spinning is essentially the process in which a roving of textile fibres is transformed into a twisted yarn by first performing a drafting and controlled elongation of the roving by which it is given the desired dimensions, which determines the yarn count of the yarn produced from it, and then giving the bundle of fibres of the roving, which have received only a slight twist in the preceding working and are still substantially parallel, an effective and necessary twist to give the yarn an adequate strength by making it pass along a path in high speed rotation between a fixed ring and a rotating spindle with the interposition of a ring driven by the spindle itself. The yarn thus twisted is collected by winding it on a bobbin, carried by the spindle, to form a spool which, when completed, is transferred for subsequent operations.
For a better understanding of the problems of and technical arrangements for the preparation of the roving and its spinning the conventional arrangement of one spinning station of a ring spinning machine will be briefly described in outline with reference to FIG. 1, bearing in mind that each spinning machine has a plurality of spinning stations across a spinning front. Each spinning front is constituted by hundreds of such stations which are driven in common, each receiving their services from motors which drive longitudinal axles and by delivery units which distribute their services along the machine.
The roving 1 comes from a device just above the spinning station which is not indicated in the drawing for simplicity, and is first introduced to the drafting unit 2. This generally consists of members for drafting the roving at linearly increasing speeds which gradually reduce it by making the fibres of which it is composed slide over one another. In FIG. 1 the drafting unit is driven via a pair of belts 3 and 4 of which the underlying belt 3 is driven to move by a knurled segment 5 of a longitudinal bar 6 in common with the adjacent spinning stations and rotating in direction of the arrow a.
The path of the lower belt 3 is approximately triangular, and determined by a common terminal bar 7 which extends longitudinally. The overlying belt 4 is freely movable and is driven to move in the direction of the arrow b by the underlying belt 3 onto which it is pressed by an overlying support in common with an adjacent spinning station. The path of the upper belt 4 is also approximately triangular and determined by a roller 8 and a fixed terminal bar 9.
Downstream of the belts 3 and 4 is located a pair of drafting rollers 11, 12 which impart the final draft to the roving, being provided with a linear speed greater than that of the preceding belts 3, 4. The lower roller is constituted by a grooved segment 13 of a longitudinal bar 11, common with the adjacent spinning stations and rotating in the direction of the arrow c. The upper counter roller 12 is idle and is also pressed by the common overlying support against the roller segment which drives it to rotate in the direction of the arrow d with the refined roving from which the yarn 20 is formed interposed between them.
The yarn 20 first passes the fixed yarn guide 21, typically in the form of a pigtail and from there to the rotating ring 22 which rotates on a fixed ring 23 carried by a common ring rail 24 continually driven in direction of the arrow e in two directional senses continually to cause the rings 23 to rise and fall and to distribute the winding of the thread into a spool 25 on the bobbin 26. The bobbin 26 is fitted on the underlying rotating spindle driven to rotate at high speeds which currently are in the region of 10,000-20,000 revolutions per minute.
Upon each rotation of the spindle, or rather of the spool 25, it pulls the yarn released from the drafting unit and winds it onto itself and generates substantially one revolution of twist of the yarn 20, which draws the small rotating ring 22 into rotation with a slight delay due to its friction with the guide ring 23. If the spindle rotates R revolutions per minute and cylinders 11, 12 release S meters of drafted roving 20 the twist T applied to one meter of yarn produced is equal to R/S.
The yarn 20 rotates in a vortex about the spool forming the so called `balloon` by the effect of the centrifugal force. The balloon is the cause of further stress on the yarn and can be limited with a containment ring 28. The twist in the yarn is caused by the rotation of the spool 25 and propagates, together with the stresses up to the point at which the yarn 20 is released from the last of the drafting cylinders 11, 12.
The point of release from the cylinders is the point of least strength of the yarn; the roving which begins to receive twist and tension is still in the state of parallel distributed fibres spaced along a web of a certain width. The relative weakness of the roving at the output of the drafting device constitutes a factor limiting the productivity of spinning machines.